How to call a function of another program directly (without API)?

Question

C++ / C#

Good afternoon. The question may be too vague. For starters, there is a AAA program (compiled and without an API) that has a graphical interface. The GUI button calls a function to add two numbers. Now the question is: is it possible to create another BBB program that will call the function of adding two numbers to the running AAA program without pressing a button, i.e. directly access the function call (the result is not necessarily displayed in the AAA program, a kind of cheat)?
Tell me what to read and in what direction to dig.

Answer 1

A daunting task. On Windows, you can try using CreateRemoteThread to create a thread in the address space of the target process, using SetThreadContext to cram function parameters into registers, and then jump to its address. True, the address still needs to be found out somehow. Well, I suspect that any antivirus program will have a tantrum from this.